The electrical alternating fields deriving from a video display unit, primarily a CRT-type unit (cathode ray tube-unit) are generated by a number of different electrical components included in the unit. It is a relatively easy matter to screen the electrical alternating fields in all directions with the exception of the forward direction immediately in front of the screen, for instance with the aid of screening plates, electrically conductive outer casings or housings, or with the aid of electrically conductive layers or coatings on a plastic outer casing. The electrical alternating field extending forwardly from the screen surface of a CRT-type video display unit is caused by variations in the acceleration voltage applied inwardly of the front glass of the video display unit.
One method of dealing with the effect of, among other things, the electrical alternating fields that are generated in front of a display screen involves the use of a transformer in the high voltage part with low internal impedance, although this solution is expensive if the result is to be effective. Another method involves coupling an external capacitor across the acceleration voltage. Although this solution will smooth the acceleration voltage, particularly at the low frequencies that fall within band I, it has been found that the solution is not effective enough to provide satisfactory equalization of the more high-frequency voltage variations found within band II.
Other methods involve the provision of an electrically conductive panel or like device in front of the picture tube. This solution requires a panel of very high conductivity, in order to be able to reduce the alternating fields in both band I and band II. Consequently, such panels are expensive and/or result in impaired synergonomical properties. For instance, the picture becomes out of focus, i.e. blurred, and the light yield is inferior than the light yield of a standard screen, i.e. the panel does not have full light transmission.
The object of the present invention is to achieve with a video display unit, and primarily a CRT-type unit, the smallest possible electrical alternating fields in the surroundings of the picture tube, and particularly in front of the screen of the unit.
Another object of the invention is to compensate for relatively high-frequency voltage ripple on the acceleration voltage of a CRT-type visual display unit, particularly within band II.